In MOS technology, the gate of a transistor is the part of the device that controls the operation of the device. The formation of the gate has thus been of prime importance in maintaining the integrity and performance of the device. The typical gate of the transistor is formed from a doped polysilicon layer overlying a gate oxide layer. Lightly doped drain (LDD) regions are formed by implanting a dopant into the substrate adjacent to the gate. The implantation forms lightly doped, very shallow source/drain regions. A low temperature oxide (LTO) is formed over the surface of the wafer and anisotropically etched away. The oxide on the sidewalls of the polysilicon and gate oxide remains after this etch step to form sidewall spacers. More heavily doped source/drain regions are implanted into the substrate adjacent to the edge of the sidewall spacers. The gate does not overlap the more heavily doped source/drain regions.
The uniformity of the etched LTO to form the sidewall spacers is critical to the transistor performance for devices in the submicron feature sizes. Etching of the LTO must be carefully controlled to achieve a precise endpoint. Overetching of the LTO is a standard technique to insure complete removal of the oxide over the heavily implanted source/drain regions.
This overetching technique of the LTO, however, creates problems. Thicker field oxide regions which separate the active areas must be initially grown. As the LTO is etched to form the sidewall spacers, a portion of the field oxide is also etched away. Initially, forming thicker field oxide regions, however, forms longer birds beaks and causes greater physical stress to the substrate from the larger volume of field oxide. The additional stress leads to lattice strain and defects in the substrate. Additionally, variations of the overetch process can change the drive current of the transistor, thus directly impacting transistor performance.
One way to insure that device performance is maintained after overetching the LTO to form sidewall oxide spacers is to form a layer between the LTO and the field oxide region. Placing a spacer etch stop under the LTO which forms the spacers will prevent overetching the field oxide region and control the precise etch endpoint. It would therefore be desirable to provide a method which produces a reliable transistor having the desired sidewall oxide spacer structure using an etch stop layer under the spacer oxide layer. It would further be desirable for such technique to be easily adapted for use with standard integrated circuit fabrication process flows without increasing the complexity of the process.